The physical properties of Kondo insulators SmB6 and FeSb2
|摘要:||硼化釤(SmB6)以及銻化鐵(FeSb2)分別為4ƒ及3d近藤絕緣體，在低溫時隨著溫度降低，電阻率會大幅度的上升進而停止發散。近年來研究指出，在近藤絕緣體SmB6中也具有拓樸性質，由ARPES實驗觀察到混成軌域造成的能隙中出現可供電子存在的量子態，進而解釋其低溫(T<6 K)的電阻率特殊行為，SmB6因而被稱為拓樸近藤絕緣體(Topological Kondo insulator)。而窄能隙半導體FeSb2為3d近藤絕緣體，由Fe的3d局域軌域以及Sb的s-p軌域形成混成軌域(hybridization band)，費米能階位於混成軌域形成的能隙之間，因此電阻率表現出半導體的行為。
在這篇論文中，我們利用化學氣相傳輸法(CVT)製作FeSb2單晶及助熔劑法(Flux method)製作SmB6單晶，量測電性及磁性以探討其物理性質。FeSb2的電阻率行為與SmB6類似，在低溫部分同樣都會趨近有限值，因此FeSb2可能為拓樸近藤絕緣體，同樣具有表面態(surface state)使電阻率停止發散；透過計算電阻率與溫度的曲線，FeSb2的兩個能隙分別為2.67 meV及37.31 meV；在60 K以上，磁性來自電子受到熱的激發躍遷過能隙大小為38.95 meV的自旋能隙(spin gap)，而在60 K以下，磁性受到磁性雜質的影響，隨著溫度下降而上升。因薄膜較易觀察到表面態，我們以脈衝雷射(PLD)製作出FeSb1.75之薄膜樣品。由電阻率的實驗結果同樣觀察到兩個能隙，並且10 K以下的行為與單晶類似趨近有限值。|
SmB6 and FeSb2 are 4ƒ and 3d Kondo insulators respectively, their resistivity increases as temperatures and saturates below 6 K. Recently, it is indicated that Kondo insulator SmB6 also exhibits topological properties that the in-gap state observed by ARPES to explain the saturation in resistivity at low temperatures. Therefore, SmB6 has been called topological Kondo insulator (TKI). In narrow band gap semiconductor FeSb2, 3d orbit of Fe hybridizes with s-p orbit of Sb to form the hybridization band. Since the Fermi level lies in the gap, the resistivity also shows the semiconductor behavior. In this thesis, SmB6 and FeSb2 crystals were prepared by chemical vapor transport (CVT) and flux method respectively for the study of physical properties by electrical and magnetic measurements. FeSb2 also shows the saturation in low temperature resistivity as similar to SmB6, showing the saturation in resistivity caused by surface state. Therefore, FeSb2 is also identified as one of TKI. Through the data of temperature dependent resistivity, two transport gaps 2.67 meV and 37.31 meV are calculated. Above 60 K, magnetism is contributed by the thermal activated electrons which jump to the spin gap of 38.95 meV. Below 60 K, there is an upturn in magnetic susceptibility which is caused by the magnetic impurity. In order to observe the surface state, thin film is the better choice. We made FeSb1.75 thin film by pulsed laser deposition (PLD). In resistivity measurements, the thin films which were deposited more than 2 hours were observed the saturation below 10 K and two transport gaps. The both behaviors are similar to single crystal of FeSb2.